1. Field of the Invention
The present invention relates to wireless networks, and more particularly, to space/time/power RF routing for such networks.
2. Related Art
Wireless local networks (WLANs) based on the IEEE 802.11 standard have proven to be popular. IEEE 802.11 is a wireless standard related to the IEEE 802.3 standard established for wired Ethernets. In contrast to wired networks, an IEEE 802.11 WLAN must conserve the limited bandwidth presented by a wireless transmission medium. Accordingly, a set of rules in the IEEE 802.11 standard is dedicated to medium access control (MAC), which governs accessing the wireless medium and sending data through it.
In an IEEE 802.11 WLAN, multiple wireless nodes or stations (STAs) such as laptops may communicate through an access point (AP) with other users of a wired local area network (LAN). To avoid medium access contention among STAs and APs, the MAC specifies a carrier sense multiple access/collision avoidance scheme (CSMA/CA) as controlled by a distribution coordination function (DCF).
Similar to Ethernet operation, the DCF first checks that the radio link is clear before transmitting. If another transmission is detected, the DCF specifies a random backoff period (DCF inter-frame space or DIFS). Only if the channel is clear after the expiration of the random time period, may a STA begin transmission. Because two STAs may communicate with a given AP but be out of range with respect to each other, the DCF also specifies a virtual carrier sense (VCS) procedure by which the wireless medium is reserved for a specified period of time for an impending transmission. For example, the VCS may be implemented by the use of request-to send (RTS) and clear-to-send (CTS) frames. A given STA would signal the beginning of a transmission with an RTS which specifies a reservation period. Although other STAs may be out of range from this RTS, the AP responds with a CTS that will be received by the remaining STAs. This CTS will communicate the reservation period to all the STAs, which update their Network Allocation Vector (NAV) accordingly. The NAV acts as a timer for the reservation period and protects data transmissions by causing the medium to be non-idle over the duration of the entire frame exchange.
While DCF with CSMA/CA reduces the possibility of collision, it does not eliminate it due to “hidden stations” reality. Further, DCF is not suited for time-sensitive applications because of the unbounded delays in the presence of congestion or interference. DCF may also under-utilize available bandwidth due to access contention. This inefficiency is increased because of the excessive overhead and the low data rate at which the preamble and PLCP header are transmitted. When this is used with RTS/CTS (Request to Send/Clear to Send) control frames, e.g., to handle the “hidden” station problem, inefficiency is increased even further.
If greater bandwidth utilization is desired, another type of medium access control called the Point Coordination Function (PCF) may be implemented. PCF specifies the use of special stations in APs denoted as point coordinators, which act to ensure contention-free (CF) service. PCF is a centrally based access control mechanism (as opposed to a distributed architecture) based on a polling and response protocol, where DCF is based on CSMA/CA. During a contention free period (CFP), a STA can only transmit after being polled by the Point Coordinator (PC). The PC may send polling frames to STAs that have requested contention-free services for their uplink traffic. If a polled STA has uplink traffic to send, it may transmit one frame for each polling frame received. The PC expects a response within a short inter-frame space (SIFS), which is shorter than a priority inter-frame space (PIFS).
However, PCF also has limitations, especially in WLANs with large numbers of STAs and APs, e.g., in an enterprise application. With today's needs, it is desirable to expand the coverage areas for 802.11 WLANs, e.g., networks such as offices are expanding in size. However, current 802.11 networks utilizing DCF and PCF are limited in their scalability to these larger networks, e.g., enterprise, as will be discussed below. Enterprise, as used herein, refers to networks having numerous APs and STAs in large physical areas.
One limitation of PCF is that since the CFP repetition rate is not dynamically variable, there is a trade-off between low latency applications requiring a fast repetition rate and an efficient use of the medium requiring slower repetition rate. Also, the start of a CF period is not exactly periodic (i.e., it can only begin when the medium is sensed to be idle). As a result, the CFP may be forced to end before serving some STAs on the polling list. Further limitations may be that all the CF-pollable APs have the same level of priority since they are simply polled by ascending Association ID (AID), and the PC has to poll all the STAs on its polling list even if there is no traffic to be sent.
In addition to governing medium access, DCF also specifies rules for the frequency scheme that may be implemented in a WLAN having multiple APs. For example, FIG. 1a shows a WLAN having coverage areas or cells 10 corresponding to four APs. To minimize contention between adjacent APs, each AP is assigned a frequency channel for its cell 10. FIG. 1a shows that the overlap of cells 10 allows complete coverage within the WLAN.
IEEE 802.11 WLAN suffers from the rigidity required by the DCF. In addition, FCC regulations limit the amount of power that can be used by the APs and STAs. The DCF rigidity and power allocation limits present severe challenges to users during network deployment and modification. Even if careful network planning is implemented, there may still be loss of bandwidth due to unpredictable circumstances such as a subscriber's movement and activity level. In addition, the overall network bandwidth is limited by non-adjacent cell interference such that cells cannot be readily reduced in size to boost bandwidth. Moreover, because frequency channels are used for interference-avoidance planning, cell bandwidth is limited to a single AP bandwidth. When external interference is present, system frequency cannot be easily changed since the frequency resource already been used for another purpose.
Network bandwidth cannot be easily allocated to areas of high demand (such as conference rooms). Finally, as with any segregated medium, the DCF rigidity leads to a loss of trunking efficiency.
For example, FIG. 1a illustrates an ideal situation where each cell 10 has a circular coverage area. However, in reality, the coverage area of each cell 10 is not a circle. For example, in an enterprise application, such as in a building with large numbers of walls and offices, numerous APs and STAs are needed to allow STAs to transfer information between each other. The walls and other barriers result in non-uniform coverage areas for each cell 10.
FIG. 1b shows coverage areas or cells 20 in a practical WLAN environment. As seen, the coverage areas are no longer uniform circles, but are irregular having areas of broader coverage (the peaks) and areas of lower coverage (the nulls). For example, long peaks 25 may correspond to long hallways in the building. Because cells 20 do not have uniform coverage, “holes” 30 exist in the network, where communication is not possible. Holes 30 do not necessarily represent areas where no frames are able to be sent and received, but rather only a small percentage of dropped frames are all that may be tolerated due to TCP/IP behavior, thereby effectively ending communication ability within that area.
A possible solution to “fill” holes 30 may be to increase the density of the APs in the WLAN, i.e., move the APs closer to each other, which requires more APs for the same outer coverage area. However, increasing the density of the APs will result in increased interference between APs and STAs, while also increasing the cost of the system. Consequently, in order to reduce interference, the transmit power of the APs must be reduced. But, this may again result in holes in the WLAN coverage due to irregular coverage “footprints” of the APs at an additional cost of a reduction in maximum throughput of the system.
Accordingly, there is a need in the art for improved techniques to enhance cell coverage, eliminate the need for cell-based frequency planning, and increase network effective bandwidth and trunking efficiency in WLANs, especially in enterprise.